1. Technical Field
The invention relates generally to varactor diodes, and more particularly to a varactor diode that has enhanced properties for RF CMOS and BiCMOS applications.
2. Background Art
Variable reactors (varactors) are essential for the design of key radio frequency (RF) CMOS and BiCMOS circuits, and are specifically used as tuning elements in voltage controlled oscillators (VCOs), phase shifters, and frequency multipliers. A varactor is a diode having a capacitance that varies as a function of applied voltage. Examples of such diodes include U.S. Pat. No. 3,396,317, “Surface-Oriented High Frequency Diode;” U.S. Pat. No. 3,634,738, “Diode Having a Voltage Variable Capacitance Characteristic and Method of Making Same;” U.S. Pat. No. 3,636,420, “Low-Capacitance Planar Varactor Diode;” and U.S. Pat. No. 3,860,945, “High Frequency Voltage-Variable Capacitor.”
In order to enhance the capacitive swing of a varactor it also known to vary the dopant concentration of one or both of the diffused electrodes of the diode such that the diffusion has a retrograde dopant profile (that is, the dopant concentration is higher at the lower portion of the diffusion region than it is in the top). These so-called “hyperabrupt” junctions greatly increase the change in varactor capacitance for a given voltage swing. See U.S. Pat. No. 3,638,300, “Forming Impurity Regions In Semiconductors;” U.S. Pat. No. 3,706,128, “Surface Barrier Diode Having a Hypersensitive N Region Forming a Hypersensitive Voltage Variable Capacitor.” U.S. Pat. No. 4,226,648, “Method of Making a Hyperabrupt Varactor Diode Utilizing Molecular Beam Epitaxy;” and U.S. Pat. No. 4,827,319, “Variable Capacity Diode With Hyperabrupt Profile and Plane Structure and the Method of Forming Same.”
In general, varactor designs must maximize a number of properties. One is “tunability,” which is the ratio between the highest and lowest capacitive values (Cmax/Cmin) over the range of applied voltages for the circuit. Another is “linearity.” There are two definitions of ‘linearity’: 1/sqrt(C) and d(lnC)/dV, where C is the voltage-dependent varactor capacitance. In the first case it is desired that 1/sqrt(C) be a straight line and the second that d(lnC)/dV be a constant, both as V varies. Yet another property is “Q,” or quality factor, which a function of the series resistance of the diode and the capacitive value of the varactor at the higher frequency ranges of the circuit. See Kannnam et al, “Design Considerations of Hyperabrupt Varactor Diodes,” IEEE Transactions of Electron Devices, Vol. ED-18, No. 2, February 1971 pp. 109-115 for a discussion of the interplay between tunability and Q.
In practice, it has proven to be difficult to simultaneously enhance tunability, linearity, and Q of a varactor when integrated into a CMOS or BiCMOS process. For example, considering the PFET source/drain junction and well as a varactor device, additional n-well implants will decrease the well resistance and increase varactor Q, but will decrease varactor tuning range by making the source/drain p-n junction depletion regions smaller.
Accordingly, a need has developed in the art for a varactor design that optimizes the tradeoffs between all of these properties, particularly when integrated into a process for forming other integrated circuit devices.